The majority of non-Hodgkins lymphomas (NHLs) are of B-cell origin and over 90% express the leukocyte antigen CD20. The chimeric anti-CD20 antibody C2B8 (Rituximab; RITUXAN®, IDEC Pharmaceuticals, San Diego, Calif.; Genentech, San Francisco, Calif.) has been extensively evaluated in patients with recurrent B-cell lymphomas and its efficacy has been proven in several clinical trials. The mechanism of action in vivo is still not clear and multiple hypotheses have been generated i.e. induction of apoptosis, antibody dependent cell-mediated cytotoxicity (ADCC), phagocytosis, complement mediated cell-lysis (CML) and cross-priming of CD8+ cytotoxic T-cells. The efficacy of anti-CD20 antibodies has been further enhanced by radiolabeling with therapeutic isotopes such as 131I (tositumomab and Iodine I 131 tositumomab; BEXXAR®) and 90Y (ibritumomab tiuxetan; ZEVALIN®).
We have previously generated engineered antibody fragments including diabodies (dimers of single-chain Fv; scFv; 55 kDa), minibodies (dimers of scFv-CH3; 80 kDa) and scFv-Fc DM (dimer of single-chain Fv-Fc, 105 kDa) with pharmacokinetics optimized for imaging in vivo. MicroPET imaging using I-124- or Cu-64-labeled fragments targeting both carcinoembryonic antigen (CEA) and IIER2 demonstrate rapid, high level tumor targeting and fast blood clearance in mice carrying human colon carcinoma or breast carcinoma xenografts have resulted in high contrast, antigen-specific images. Fluorodeoxyglucose F18, (2-deoxy-2-[18F]fluoro-D-glucose) ([F-18]-FDG tracer) is currently standard for clinical PET imaging, but for lymphomas utility can be limited in cases of indolent disease with low metabolic activity. An imaging agent directed against a cell-surface target could provide complementary information.
Here, we describe the generation of anti-CD20 rituximab minibody that demonstrates rapid, specific localization to CD20 expressing tumors in a murine model, and are promising as imaging agents for detection of indolent NHLs. Anti-CD20 antibody fragments are useful for in vivo imaging and therapy for CD20 expressing cancers such as NHL, as well as for other CD20 targeted therapies, such as those used to treat autoimmune disease (RA, SLE, Sjogren syndrome, cold autoimmune hemolytic anemia) and graft vs. host disease.